In a blowing machine, in particular a stretch blowing machine for plastic bottles, prefabricated preforms are conveyed along a conveyor line by a conveyor chain along heating stations of a heating module before they are processed in the blow wheel or the blowing station. Each preform is already formed with an opening which at least essentially corresponds to the opening of the finished bottle. On the conveyor chain heating mandrels are exchangeably held, which are adapted to the opening of the preform and loaded with preforms on entering the heating module, and which often rotate the preforms when passing through the heating module, and are then unloaded of the preforms, before said preforms are processed in the blowing station. Each preform is held at the opening by the heating mandrel inserted in it. Since many different types of container can be manufactured in the blowing machine, for example bottles, which require different openings (opening diameter, opening length, opening shape and retaining ring), the blowing machine for preforms must be retooled with a different opening, at least with regard to the heating mandrels. Apart from heating mandrels there are other fittings on the conveyor chain, such as shielding plates, which are often also to be changed during retooling. The shielding plate has a through passage for the heating mandrel or the preform, whereby the through passage should enclose the preform as tightly as possible to shield the opening in the heating module from unwanted thermal exposure. For preforms with a different opening length only the heating mandrels need to be optionally changed. For preforms with a different opening diameter usually the shielding plates also have to be changed. In practice these changing processes have so far been carried out manually and individually by operators, and are laborious and time-consuming, because there are up to 600 heating mandrels and shielding plates on the conveyor chain. The optionally high residual thermal stress is unpleasant for the operator and provokes erroneous changeovers. In addition, a manual changeover results in a very long downtime in production for the blowing machine or a complete plant in which the blowing machine is a component.